Movement of Substances Across a Plasma Membrane
The plasma membrane controls what enters and leaves a cell. This chapter explains its fluid mosaic structure and the three ways substances cross it, diffusion, osmosis and active transport, plus how these apply in living organisms and daily life.
Getting the definitions and the direction of movement exactly right is where most marks are won.
The membrane itself is more than a passive barrier. Beyond the phospholipid bilayer, embedded and peripheral proteins act as channels, carriers and receptors, while glycoproteins and glycolipids on the outer surface let a cell recognise hormones, antigens and neighbouring cells.
This selective structure is why the membrane is described as partially permeable, small non-polar molecules such as oxygen and carbon dioxide diffuse straight through the lipid bilayer, whereas larger or charged particles such as glucose and ions must pass through a specific protein channel or carrier instead.
The rate at which particles cross the membrane is not fixed. It rises with a steeper concentration gradient, a larger surface area, a smaller particle size and a higher temperature, since molecules then move faster and collide with the membrane more often.
SPM questions often ask candidates to explain how changing one of these factors affects the rate, so learn each factor together with the reason it speeds up or slows down diffusion, osmosis or active transport, rather than only memorising the list.
This chapter is assessed heavily through Paper 2 structured questions built around a labelled diagram, a set of experimental results or a short scenario, and through Paper 1 items testing definitions and factors. A strong answer states the correct term, the direction of movement and the reason together, since examiners award marks for each linked idea rather than for the term alone.
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Content standards in this chapter
- 3.1 Structure of the Plasma Membrane
- 3.2 Concept of Movement of Substances Across a Plasma Membrane
- 3.3 Movement of Substances Across a Plasma Membrane in Living Organisms
- 3.4 Movement of Substances Across a Plasma Membrane and its Application in Daily Life
Key concepts
- Fluid mosaic model
- A phospholipid bilayer with proteins scattered through it; it is partially (selectively) permeable.
- Diffusion
- Net movement of particles from high to low concentration, down a concentration gradient, without energy.
- Osmosis
- Net movement of water molecules from a less concentrated (dilute) to a more concentrated solution across a partially permeable membrane.
- Active transport
- Movement of substances against the concentration gradient, using energy from respiration and carrier proteins.
- Effects on cells
- In hypotonic solution animal cells burst (haemolysis) and plant cells become turgid; in hypertonic solution animal cells shrink (crenation) and plant cells plasmolyse.
- Applications
- Osmosis and diffusion explain wilting, food preservation by salting, and root absorption of minerals by active transport.
- Membrane transport proteins
- Channel proteins form a fixed, water-filled pore that lets specific ions or small polar molecules diffuse straight through the membrane. Carrier proteins instead bind the particle, change shape and release it on the other side; they can work passively in facilitated diffusion or, using ATP, actively pump a substance against its concentration gradient.
- Factors affecting rate of movement
- The rate of diffusion, osmosis and active transport increases with a steeper concentration gradient, a larger surface area, a smaller particle size and a higher temperature, because particles then have more kinetic energy and collide with the membrane more often; active transport is further limited by the amount of ATP and carrier proteins available.
- Water potential
- Water potential measures how readily water molecules move out of a solution; pure water has the highest water potential, and dissolving a solute always lowers it. Water moves by osmosis from a region of higher water potential to a region of lower water potential, which is the same as saying it moves from a less concentrated to a more concentrated solution.
- Osmosis and daily-life technology
- A kidney dialysis machine uses diffusion across a partially permeable membrane to remove urea and excess ions from a patient's blood when the kidneys fail. Intravenous drip fluids must be isotonic to blood plasma; an incorrectly concentrated drip would make red blood cells swell and burst or shrink by osmosis.
- Movement of substances in living organisms
- The three mechanisms act side by side in real organisms: gas exchange at the alveoli and in root hairs happens by diffusion, water enters root hair cells and red blood cells adjust volume by osmosis, and reabsorption of glucose in the kidney tubule and mineral-ion uptake in roots depend on active transport.
- Osmoregulation link
- This chapter's principles underpin the process of osmoregulation, covered in more detail elsewhere: freshwater organisms constantly gain water by osmosis and must expel it, while marine organisms in a saltier environment tend to lose water and must conserve it, so the surrounding environment always matters when predicting the direction of osmosis.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Defining osmosis precisely and predicting whether a cell gains or loses water.
- Explaining visking tubing or potato-strip experiment results.
- Comparing diffusion, osmosis and active transport in a table.
- Explaining, using water potential or concentration language, why water moves into or out of a cell placed in a stated solution, and stating the direction and the reason together for full marks.
- Interpreting a graph or data table that shows how surface area, temperature or concentration affects the rate of diffusion, osmosis or active transport, and identifying which variable was changed.
- Applying active transport correctly to mineral-ion or glucose absorption against a concentration gradient, linking it to energy from respiration and to carrier proteins rather than channel proteins.
- Recognising that osmosis is a special case of diffusion that applies only to water molecules moving across a partially permeable membrane, rather than treating the two terms as interchangeable.
Common mistakes
What students write: Saying osmosis is the movement of water from high to low concentration.
What earns the mark: Osmosis moves WATER from a less concentrated (more dilute) to a more concentrated solution, i.e. down the water potential gradient.
What students write: Forgetting that active transport needs energy.
What earns the mark: Active transport moves substances against the gradient and requires energy (ATP) from respiration and carrier proteins.
What students write: Using 'turgid' and 'plasmolysed' for animal cells.
What earns the mark: Turgid and plasmolysed describe plant cells; animal cells undergo haemolysis (burst) or crenation (shrink).
What students write: Writing that diffusion needs a membrane.
What earns the mark: Diffusion happens in any medium; it does not require a partially permeable membrane, unlike osmosis.
What students write: Believing a cell in an isotonic solution has stopped exchanging water altogether.
What earns the mark: Water still moves both ways across the membrane in an isotonic solution; there is simply no net movement, because water enters and leaves at the same rate, so the cell's size stays constant.
What students write: Calling facilitated diffusion a form of active transport because it uses a protein.
What earns the mark: Facilitated diffusion uses a channel or carrier protein but still moves particles down the concentration gradient without using energy, so it remains a passive process, unlike active transport.
What students write: Assuming a larger cell or organ always absorbs substances faster than a smaller one.
What earns the mark: Absorption depends on the surface area to volume ratio, not size alone; smaller structures such as root hairs or villi often have a higher ratio and so absorb relatively faster.
What students write: Writing that a higher temperature always speeds up every membrane process without limit.
What earns the mark: Diffusion and osmosis speed up as temperature rises because particles gain kinetic energy, but active transport depends on carrier proteins, which are proteins that denature at high temperatures, so an excessive temperature can reduce rather than raise the rate.
What students write: Thinking active transport only happens in plant roots.
What earns the mark: Active transport occurs in many places in the body and in plants, root hair cells absorbing mineral ions, kidney tubules reabsorbing glucose, and nerve cells running the sodium-potassium pump, wherever a substance must move against its own concentration gradient.
Study this chapter
Processes in this chapter
Structures in this chapter
Experiments in this chapter
Frequently asked questions
What is osmosis in simple terms?
How is active transport different from diffusion?
Why does a plant wilt when the soil is dry or too salty?
What is water potential and how does it relate to osmosis?
How do channel proteins and carrier proteins differ?
Why is osmosis considered a special case of diffusion?
Source:SRC-DSKP-EN, SRC-FORMAT
Related
Active transport
ReadFacilitated diffusion
ReadPlasma membrane structure (fluid mosaic model)
ReadEffects of Solution Concentration on Cells
ReadHypotonic solution
ReadDiffusion
ReadConcept of Movement of Substances Across a Plasma Membrane
ReadMovement of Substances Across a Plasma Membrane in Living Organisms
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